It is previously known to use beam-shaped load cells or force transducers based on resistive strain gauges in electronic weighing equipment of various types. The reason for this is that this type of load cell has proved to be very reliable and they have also the ability to maintain calibration and reproduction data for a long time and under severe weighing conditions. The beam-shaped load cells are also comparatively small in size, which makes them suitable for built-in and low-profile weighing applications. The load cells are usually cylindrical and when they are installed they can therefore often replace an existing shaft or any other machine element in order to sense a strain or measure a load without requiring any modification of the equipment in which they are installed.
A significant quality of shear beam type load cells is the ability to withstand side forces, that is, forces acting in directions other than in the measuring direction of the load cell. The load cells do not require any side supporting means, they can withstand side forces up to 100-200% of the nominal load value.
However, a load cell can also be affected by other undesired forces and as is usual in measuring applications it is the temperature and specifically temperature variations that cause measuring errors. Of course the strain gauges which are used in the load cell have themselves a certain temperature dependence caused by the fact that the measuring element and the resistive wire have different thermal expansion coefficients. If, however, at least four strain gauges are used and if they are of the same kind and glued on the same material, then the changes in resistance caused by the temperature variations will be the same for each of the strain gauges. As the strain gauges are arranged in a bridge circuit, the balance of the bridge will not be changed.
Undesired load forces affecting the measuring accuracy are also caused by, for instance, temperature expansions in the weighing container or platform. For that reason the load cell is usually provided with a bearing or other type of embodiment for eliminating these types of disturbing forces, such as side forces or twisting moments of torsion.
Swedish patent 82 01365-7 discloses a cylinder-shaped load cell with such a built-in bearing for permitting a side movement of the measuring body on a plane base support member. The load cell can be positioned directly on a foundation or other base support member in such a manner that it is permitted to move in a side direction. By means of such a built-in bearing the load cell can be loaded via a loading bracket without any additional bearings required.
In many applications, however, these types of movements on a plane foundation or base support member are not permitted by the actual design of the weighing equipment. Furthermore, the bearing built into the cylinder-shaped measuring body makes this type of load-cell comparatively expensive. The built-in bearing, in the form of, for instance, slide bearing or a roll bearing, must be designed with a high degree of accuracy and withstand the often very heavy load forces which are applied to this type of load cell.
Another type of prior art load cell is disclosed in U.S. Pat. No. 3,960,228. This load cell has a free deflectable beam secured to a foundation or other mounting means in such a manner that the free deflectable weigh end of the beam is suitably supported for receiving a vertical load force to be measured. The strain gauges are positioned on opposite sides of the beam and orientated substantially at a 45.degree. angle to the longitudinal neutral axis of the beam. A recess is formed in the free end of the deflectable beam in the load-measuring direction so that the vertical force of the load is applied on a load bearing surface within the recess. The load bearing surface within the recess is located close to the longitudinal neutral axis of the shear beam so that the moment arm and the twisting moment of an adverse side force affecting the shear beam are minimized. Thus, in this type of load cell the measurement inaccuracy, due to imperfect positioning of the strain gauges on opposite sides of the shear beam, is reduced.
Also, in the case of a precise positioning of the two opposite strain gauges, this arrangement does not solve the problems, however, due to thermal expansion in the weighing container or weighing platform. The shear beam is secured to a foundation or the like and is not permitted any movement for compensating for such thermal expansions.
Swedish patent 311 573 discloses a load cell in the form of a shear beam, in which the free deflectable end of the shear beam is fastened to one end of a second beam extending parallel to and freely at the side of the first beam. The second beam is shorter than the first beam and, the free end of the second beam provides the receiving surface for the load force to be measured. This second beam is preferably made as a sleeve member freely enclosing the first beam.
One important advantage of such an arrangement is the fact that the load cell is substantially insensitive to any change in point-of-load application. Specifically, any changes of the point-of-load application due to thermal expansion in a weighing container or a weighing foundation affect the result of the measurement only to a very small degree.
Even if this type of load cell with a second sleeve enclosing the first shear beam has this important advantage it should be understood that this type of load cell is more expensive in construction. Therefore, a load cell is desired which is more simple in construction.